Advice on making these machicolations
For this kind of pattern repeating around a circle, Array and Simple Deform modifiers can often be a good solution:


Create your pattern, as it is symmetrical use a Mirror modifier.





Give it an Array modifier to repeat it. Choose the Count you need, enable the Merge and First Last options.





Give it a Simple Deform modifier / Bend mode, choose an Angle of 360°. If you see that it twists strangely, you need to apply the rotation of the object.





Give it a Subdivision Surface modifier and Shading > Smooth it in the T panel. Now you can work the mesh as much as you want until you're satisfied. When you think it's ok, apply the modifiers and merge the first and last vertices with a Remove Doubles.